An audio system for generating hyper-directional sound beams in the audible range is known. The audio system employs an array of acoustic transducers to project through the air an ultrasonic carrier wave modulated with signals representing audible sounds. Due to non-linear propagation characteristics of transmission media such as air when excited by finite amplitude ultrasonic waves, the modulated ultrasound self demodulates on passage through the transmission medium, creating endfire virtual sources along a selected projection path to produce a hyper-directional sound beam in the audible range. Although the sound beam is demodulated with relatively high levels of harmonic and intermodulation distortions it is possible to obtain a relatively linearized characteristic by pre-distorting or pre-conditioning the audible signal before modulation.
In particular, it is noted that, when the primary wave is a modulated carrier, the sound generated (upon demodulation) by the secondary (modulating) wave is proportional to the second time derivative of the square of the modulation envelope. This results in high levels of harmonic distortion in the sound generated. To address this, D. T. Blackstock (refer “Audio Application of the Parametric Array,” J. Acoust. Soc. Am., Vol 102 pp 3106(A), 1997) and others (refer T. Kamakura, M. Yoneyama, K. Ikegaya, “Developments of parametric loudspeaker for practical use”, 10th Int. Symp. Nonlin. Acous., pp. 147–150, 1984 and T. D. Kite, J. T. Post and M. F. Hamilton, “Parametric array in air: distortion reduction by preprocessing”, Proc. Int. Conf. Acous./Acous. Soc. Am, vol. 2, pp. 1091–1092, June 1998.) suggest methods of improving distortion in the demodulated signal. Blackstock proposes integrating the original signal twice and taking the square root thereof; to anticipate the demodulation function and thus remove the distortion resulting from demodulation. This is shown in the following equation, where f(t) is the audio signal and E(t) is the signal provided to the modulator:E(t)=(1+∫∫ƒ(t)dt2)1/2  (1)
However, due to the square root operation, this preprocessing approach generates an infinite number of harmonics. Harmonic distortion will only be removed if all of these harmonics are reproduced. Therefore, the amount of distortion of the demodulated signal is directly related to the bandwidth of the device, and the method requires bandwidth-intensive ultrasonic paths and emitters to get optimal performance.
The sound beams produced by the above technique may be focused, steered or projected in a defined area or direction. Reflection may take place when the modulated carrier wave encounters an object that absorbs energy at ultrasonic frequencies but reflects energy at audio frequencies.
Advantages of using an ultrasonic carrier wave to deliver audio include the highly directional nature of the modulated ultrasonic wave, the fact that the carrier wave is steerable (for example by providing reflective surfaces), and also that the signal is not audible prior to demodulation. By proper application of these advantages, audio can be delivered to specific locations, from where the audio appears to originate. A general discussion of the transmission of audio signals can be found in European published patent application no. EP 973 152.
However, other technical challenges remain in the use of ultrasonic technology for delivering audio. For example, the fidelity of the demodulated audio signal can still be improved. In particular, the delivery of adequate power at low frequencies is a problem. Human hearing is more sensitive to stimuli having middle frequency components (i.e. 3–4 kHz) than low frequency components (i.e. “bass,” below 500 Hz). To perceive lower frequency sounds at the same loudness as at the middle frequencies, it is necessary to generate higher sound pressure levels at the lower frequency.
One useful application for the sound beams is in advertising. A problem with simultaneous audio-broadcasting of advertising material is that it creates noise pollution in public places such as shopping malls, public transport stations (bus stops and train terminals), conference and exhibition halls and the like. This may create a relatively high level of interference and confusion for the listener who hears mixed signals from different broadcasting sources.
A parametric audio system incorporating a steering function that uses a phased array technique is described in WO01/52437 (Frank Joseph Pompei). The latter system includes a delay circuit to apply a relative phase shift or delay across all frequencies of the modulated carrier signal to steer, focus, or shape ultrasonic beams generated by the acoustic transducer array.